Alkaline Earth Metal Ions Dynamics in Mixed Ionic-Electronic Conductors of Graphite Intercalation Compounds

Materials that effectively facilitate the transport of ionic and electronic charges are crucial for advancing technological innovations in next-generation energy storage devices. This work proposed a new class of high-performance mixed ionic-electronic conductors (MIECs) in graphite intercalation compounds with the composition XC₆ (X = {Ca, Sr, and Ba}) using molecular dynamics based on machine learning force fields combined with first-principles calculations. The calculated mean squared displacement and radial distribution functions indicate that CaC₆, SrC₆, and BaC₆ transition to the superionic state at temperatures of 1500, 1800, and 2100 K, respectively. Alkaline earth metal cations can diffuse through two pathways via the vacancy migration mechanism: they can either move across carbon-carbon covalent bonds or migrate to the position above a carbon atom, subsequently diffusing to the center of an adjacent carbon hexagon. Additionally, these materials exhibit high ionic conductivity and excellent thermal and mechanical stability. The results suggest that the introduction of defects effectively regulates the superionic transition temperature, and CaC₆ with 10% defects achieves a conductivity of approximately 0.05 S cm^-1 at 550 K. We provide a new prospect from the perspective of ion dynamics to design advanced MIECs as high-temperature-resistant electrodes and interface improvement materials.